Hi,

I am using the AIREBO potential to simulate graphite (5x5x5 unit cells) with a LAMMPS version from 21Jun12. When I try to find the equilibrium lattice parameter through energy minimization, I get an in-plane lattice parameter of 1.61 Å even though it should be 1.396 for this potential. Varying the lattice parameter and calculating the energy to get an energy vs parameter curve gives the same result (total energy lowest at 1.61). Switching off the LJ and torsion terms gives an in-plane parameter of 1.398, so I suspect the problem is in the interplanar interactions, but I am using the recommended LJ cutoff of 3*sigma. Varying the cutoff does not help, neither does changing to 10x10x10 unit cells. Please help me spot the error!

Input file:

units metal

boundary p p p

atom_style atomic

#Positions input file

read_data data.graphite

#Force field

pair_style airebo 3.0 1 1

pair_coeff * * CH.airebo C

#Minimisation

fix 1 all box/relax tri 0.0

min_style cg

min_modify line quadratic

minimize 1e-20 1e-10 10000 100000

write_restart test1.restart

The configuration file data.graphite is too long to post here, but it’s ordinary graphite with lattice parameters given by experiments (a=1.42, c=3.354). The cell looks like this:

0.0 12.295 xlo xhi

0.0 10.6477823395 ylo yhi

0.0 16.77 zlo zhi

-6.1475 0.0 0.0 xy xz yz

Best regards,

Edit

Edit Helgee, Ph.D. student

Department of Applied Physics

Chalmers Univ. of Technology

S-412 96 Goteborg, Sweden

Tel: +46 31 7723195